Hybrid sealing composite for flat solid oxide fuel cell stack

ABSTRACT

The present invention provides a hybrid composite sealant, as a sealing material for a planar type solid oxide fuel cell stack, having a matrix of a glass composition, wherein a surface layer reinforced with platelet reinforcement particles is laminated on either one or both surfaces of an inner layer reinforced with fibrous reinforcement particles. Accordingly, by applying the composite sealant of the present invention to the solid oxide fuel cell stack, excellent gas-tightness of the stack can be obtained even under low coupling pressure, thermal cycling durability can be enhanced due to low coupling strength with a contact surface of an object to be sealed, stack disassembly and maintenance can be facilitated when parts within the stack are disabled, and stack stability as well as stack performance can be maintained under a pressurized operation condition where pressure differentials between the inside and outside of the stack reach to 5 atmospheric pressures (0.5 MPa).

TECHNICAL FIELD

The present invention relates to a hybrid composite sealant for planartype solid oxide fuel cell stacks, and more particularly, to a hybridcomposite sealant which can implement gas tightness of a stack even in astack under a pressurizing operation where pressure differentials existbetween the inside and outside of a fuel cell stack, can remarkablyreduce an interfacial adhesion strength between a interconnector and anelectrolyte contacted by the sealant material so as to prevent thedirect failure of the sealant material itself due to thermal stress, canenhance stability during thermal cycling and long-term stability, andcan permit unit cell replacement within a stack or a interconnectorreplacement and maintenance.

BACKGROUND ART

A sealing material in a planar type solid oxide fuel cell, beinginserted between a interconnector and an electrolyte, is used toseparate a fuel gas supplied to an anode of the cell from the airsupplied to a cathode of the cell. Currently, various kinds of sealingmaterials have been developed. Among them, glass-ceramic compositesealants have the best quality in gas tightness. However, it isdifficult to carry out a pressurizing operation using a compositesealant composed only of a glass matrix phase.

In order to solve this problem, a stack may be installed in apressurizing container to operate in a pressurizing operation. In thiscase, however, the pressurizing container may undesirably increase thebulk of the stack itself and require additional installation cost.

Accordingly, a sealing material is needed which is capable of securingthe gas-tightness of the stack without requiring an additionalpressurizing container.

DISCLOSURE OF THE INVENTION Technical Problem

Therefore, it is an object of the present invention to provide a sealingmaterial which is capable of securing the gas tightness of a stackwithout an additional pressurizing container during a pressurizingoperation, which overcomes the limitations and disadvantages associatedwith the above-mentioned problem.

It is another object of the present invention to provide a sealingmaterial, which can control the direct fracturing of a sealing materialitself due to thermal stress, enhance stability in thermal cycling, andpermit cell replacement within the stack or interconnector replacementand maintenance.

Technical Solution

To achieve these and other advantages and in accordance with an aspectof the present invention, there is provided a hybrid composite sealant,as a sealing material for a planar type solid oxide fuel cell stackhaving a matrix of a glass composition, in which a surface layerreinforced with platelet reinforcement particles is laminated ontoeither one or both surfaces of an inner layer reinforced with fibrousreinforcement particles.

In order to achieve these and other advantages and in accordance withanother aspect of the present invention, there is provided a hybridcomposite sealant, as a sealing material for a planar type solid oxidefuel cell stack, which includes platelet reinforcement particles in aglass matrix in a surface layer portion of the sealing material.

In order to achieve these and other advantages and in accordance withstill another aspect of the present invention, there is provided ahybrid composite sealant, as a sealing material for a planar type solidoxide fuel cell stack, which includes fibrous reinforcement particles ina glass matrix in an inner layer portion of the sealing material,wherein a surface layer portion of either one or both sides of the innerlayer portion includes platelet reinforcement particles in a glassmatrix, and wherein the inner layer portion and the surface layerportions are integrally formed.

EFFECT OF THE INVENTION

Even though the hybrid composite sealant of the present invention hastwo layers of excellent mechanical strength and fracture toughness, afiber-reinforced glass matrix composite has higher mechanical strengthand fracture toughness than a platelet-reinforced glass matrixcomposite. Platelet reinforcement particles added into the surfacelayers reduce the content of the glass matrix on the mating surface,resulting in remarkably reduced interfacial adhesion strength.Accordingly, the interconnector-sealing material interface can bedeformed under external stress, thus to prevent damage to a unit cell aswell as to the sealing material. Furthermore, according to the presentinvention, stability in thermal cycling of a stack can be enhanced andalso high gas-tightness can be obtained in a stack having a large area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a sandwich-type hybrid compositesealant having a glass matrix composite inner layer in which are addedfibrous particles as reinforcement and glass matrix composite surfacelayers in which are added platelet particles as reinforcement.

FIG. 2 is a schematic diagram showing area fractions occupied byreinforcement particles in a contact interface when the sealing materialis contacted with a surface of an interconnector or electrolyte.

MODES FOR CARRYING OUT THE PREFERRED EMBODIMENTS

The hybrid composite sealing material of the present invention can be asandwich-type hybrid structure (referring to FIG. 1), in which an innercomposite layer has fibrous reinforcement particles and surfacecomposite layers on both sides of the inner composite layer haveplatelet reinforcement particles, or can be a laminated structure havinga surface composite layer on either one side only, depending on thematerials to be contacted.

Herein, the basis for determining whether to use a surface compositelayer depends on whether or not the main body of the sealing material issusceptible to damage or failure when a thermal stress is developed in asealing material with high interfacial adhesion strength to the matingsurfaces. If the interfacial adhesion strength with the mating surfacesis not too high, then composite layers including fibrous reinforcementparticles can be used.

In accordance with the present invention, a compressive hybrid compositesealant can maintain excellent gas-tightness of a stack due to its glassmatrix phase. The inner layer making up the majority of the sealingmaterial can maintain high mechanical stability due to its fibrousreinforcement particles. The surface layers including plateletreinforcement particles can maintain a relatively low interfacialadhesion strength to the mating surfaces.

When applying the hybrid sealing material of the present invention to astack structure, since the sealing material itself maintains highmechanical stability while its interfacial strength is relatively lowcompared to the mechanical strength of the sealing material, a stack canbe manufactured with high fracture resistance to thermal stress causedby thermal cycling of the stack or non-uniform distribution oftemperature in a stack of a large area and the like.

In more detail, under a condition of thermal stress occurring inside thestack, since deformation or fracture can preferentially occur on asurface portion of a hybrid sealing material, that is, on an interfacebetween the interconnector and the sealing material or an interfacebetween the electrolyte and the sealing material, damage or fracture ofthe sealing material itself can be prevented, and unit cells forming thestack can be protected, thereby improving stack reliability and enablingthe repair of a stack when necessary.

When a compressive sealing material is applied, stack coupling pressureis applied from the exterior all the time and the distribution of theglass matrix varies according to the distribution of the couplingpressure. However, since the capillary diameter of fiber network of theinner layer is smaller than that of the outer surface layer, thecompressed hybrid composite sealant of the present invention has thegradient structure of capillary diameter of reinforcement particlenetwork, in which the glass matrix moves toward the inner layer due tothe difference in capillary pressure of glass. Accordingly, thecompressive hybrid composite sealant of the present invention cancontain relatively high glass content included in the composite sealant,can easily obtain gas-tightness in a stack of large area, and can resistfatigue cracking during a reheating process in thermal cyclingoperation.

Description will now be given in detail of the preferred embodiments ofthe hybrid composite sealant according to the present invention.

The present invention relates to the composition of a glass-based hybridcomposite sealant which is capable of pressurized operation in a planartype solid electrolyte fuel cell stack, and exhibits a structure ofhybrid composite sealant which is capable of thermal cycling operationas well as pressurized operation of the stack.

The glass matrix phase composite sealant is inserted between theinterconnector and the electrolyte so as to secure gas-tightness of thestack by utilizing interfacial adhesion of the glass matrix phase.Composition of the glass matrix phase used in the present invention mayinclude one of B₂O₃—SiO₂, Al₂O₃—B₂O₃—SiO₂, and CaO—B₂O₃—SiO₂. However,the content of the present invention is not limited thereto. Variousshapes of reinforcement particles are added into the glass matrix phasecomposite sealant, thereby enhancing thermomechanical properties of theglass matrix phase of very low mechanical strength and fracturetoughness, and accordingly enhancing stack stability and reliability.

When a planar type solid electrolyte fuel cell stack is operated byusing fuel and air in a pressurized state, the method for assembling thestack by inserting compressive sealing materials and applying mechanicalpressure from the exterior not only facilitates the manufacturingprocess of the stack in the simplest way, but enhances stack stability.However, mechanical pressure applied from the exterior causes thesealing materials to be deformed at a high temperature, and causes theinterface adhesion strength to the mating surfaces of the connector orthe electrolyte to be increased. The various shapes of the reinforcementparticles not only have different influences upon the mechanicalproperties of the glass matrix, but also have a big impact on theinterfacial adhesion strength to the mating surfaces. Therefore, theshape of the reinforcement particles is an important factor indetermining the stability of the sealing material and the stack.

The interfacial strength of the composite sealant to the mating surfaceshas a tendency to be controlled by the area fraction of the glass matrixwhich is directly exposed to the mating surfaces. The shape and contentof the reinforcement particles added into the composite sealant can be avery important design parameter for controlling the interfacial adhesionstrength.

The shapes of the reinforcement particles can be largely divided into aparticulate shape which is close to a spherical shape, a fibrous shapewhich has a length much longer than its diameter, and a platelet shapewhich has a diameter much greater than its height.

When a planar type solid electrolyte fuel cell stack is constructedusing a compressive sealing material, a reinforcement particles havinggeometric anisotropy undergoes preferential orientation in the presenceof the mechanical pressure applied from the exterior for constructionand coupling of the stack so as to minimize the stress applied to theunit area of the particle. When mechanical pressure is applied from theexterior to the compressive composite sealants containing the identicalcontent of reinforcement particles, the reinforcement particles tend tobe placed with the most stable surface aligned perpendicular to thedirection of the pressure. Accordingly, the shape of the reinforcementparticles and the orientation thereof influence the area fractioncontacted with the mating surfaces to be sealed.

If the reinforcement particle has an isotropic shape which is close to aspherical shape, the reinforcement particle makes a point contact at apoint with the contact surface. If the reinforcement particle is afibrous shape with long length, the reinforcement particle tends to makea line contact in the form of a line segment with the contact interface.And if the reinforcement particle is a platelet, the reinforcementparticle makes a face-to-face contact with the contact interface(Referring to FIG. 2).

According to the shape and orientation of the reinforcement particles,if the contact area fraction increases, the area fraction of the glassmatrix that directly contacts with the object to be sealed would benaturally reduced. Accordingly, interfacial adhesion strength may bereduced. Therefore, the contact area fraction occupied by the glassmatrix in the contact interface has a much lower contact fraction when aplatelet reinforcement is added, compared to when a particulate or afibrous reinforcement is added, lowering the interfacial adhesionstrength.

Even though the shape of the reinforcement particles added into thecomposite sealant is an important factor in determining the interfacialstrength at the contact interface, the thermomechanical properties ofthe reinforcement particles also play an important role in sealingperformance. Fracture toughness of the glass matrix phase compositesealant shows a tendency to increase when platelet reinforcementparticles rather than isotropic reinforcement particles are added, andto greatly increase when fibrous reinforcement particles rather thanplatelet are added. Therefore, it is natural to add the fibrousparticles into the glass matrix for obtaining a thermomechanicallystable sealing material. However, correspondingly, it is inevitable tohave a relatively high interfacial adhesion strength. If the mechanicalstrength of the sealing material including the fibrous reinforcementparticles is greater than its interfacial adhesion strength, deformationor fracture due to thermal stress will be concentrated at the interface,but if not, it cannot avoid damaging the sealing material itself.

In the composite sealant of the present invention which includes thefibrous reinforcement particles having relatively high interfacialstrength with respect to the glass matrix, the fibrous reinforcementparticles are in the range of 0.5-2 μm in diameter, and the aspect ratioof length to diameter of the fibrous reinforcement particles is in therange of 5-100. The longer the length of the fibrous reinforcementparticles, the more increased the aggregation phenomenon among the fiberreinforcement particles, and accordingly, it shows a tendency toincrease the size of coarse residual pores and the frequency ofoccurrence thereof. Accordingly, the actual mechanical strength andfracture toughness of the composite sealant including the fibrousreinforcement particles may have a very low value due to the influenceof fiber clusters or coarse residual pores. The actual reduction inmechanical strength due to these process defects makes it difficult tosatisfy the requirements of the compressive sealing material, in whichthe interfacial strength should always be lower than the mechanicalstrength of the sealing material itself. Thus, thermal stress occurringin the stack may cause fracturing of the entire interface and sealingmaterial, rather than fracture along the interface.

Therefore, a hybrid composite sealant, which is capable of maintainingthe mechanical strength and fracture toughness of the composite sealantitself at a proper level and of maintaining interfacial strength at thecontact interface with the surface of the object to be sealed at theminimum, can provide durability and reliability of the stack as well asan excellent thermal cycling stability as a compressive sealingmaterial.

The hybrid composite sealant of the present invention includes afiber-reinforced composite having excellent mechanical properties and aplatelet-reinforced composite having a relatively low fraction of glassmatrix by preferential orientation of reinforcement particles, and inmore detail, a sandwich-type hybrid structure having platelet-reinforcedcomposites on both surfaces of a fiber-reinforced composite, or alaminated-type hybrid structure having a platelet-reinforced compositeon only one surface thereof.

Herein, the content of platelet reinforcement particles within thesurface layer is in the range of 5˜60 volume %, the thickness of theplatelet reinforcement particles is in the range of 0.2˜1 μm and theaspect ratio of diameter to thickness is in the range of 5˜50. Thecontent of fiber reinforcement particles within the inner layer is inthe range of 5˜55 volume %, the diameter of the fiber reinforcementparticles is in the range of 0.5˜2 μm and the aspect ratio of diameterto thickness is in the range of 5˜100. The thickness of the surfacelayer is more than 10 μm and the thickness ratio of the surface layerwith respect to the inner layer is in the range of 5˜50.

The compressive sealing material of the present invention includes acontent of reinforcement particles of which the network can containexcessive amount of glass matrix even at close packing of reinforcementparticles under mechanical pressure applied from the exterior for stackcoupling. Even though some excess glass matrix is present, thecompressive sealing material of the present invention may furtherinclude 5˜30 volume % of isotropic particulate particles in the fiberreinforced composite layer so as to prevent the glass matrix phase frommoving toward a surface portion of the sealing material due to viscousflow. Accordingly, in the capillary diameter of the network structure ofreinforcement particles for determining the distribution of the glassmatrix phase, the diameter of the capillary diameter of the inner layeris finer than that of the surface layer. And, the isotropic particulateslarger than the particulate reinforcement particles included in theinner layer can be added into the outer surface layers, and the surfacelayers may include less isotropic particulate reinforcement particlescontent than that of the inner layer.

EMBODIMENT Alumina Fiber Alumina-Platelet Alumino-Borosilicate GlassHybrid Composite Sealant

As shown in FIG. 1, the hybrid composite sealant may include an innerlayer having alumina fiber, alumina particulate, and borosilicate glass,and a surface layer having alumina platelet, alumina particulate, andborosilicate glass.

According to the embodiment, the inner layer was prepared by using 35,5, and 60 volume % of alumina fiber (Rath 97, Rath Co., Germany),alumina particulate (ALM 43, Sumitomo Co., Japan), and borosilicateglass (Pyrex glass, Iwaki Co., Japan) powders, respectively. 28 g ofalumina fiber powder, 4 g of alumina particulate powder, 26 g of glasspowder, and 1.2 g of acrylic binder (Elvacite 2045, Union Carbide Co.,USA) were added into 140 g of ethyl alcohol and uniformly mixed throughball milling for 24 hours. After the uniformly mixed slurry was driedfor 24 hours at a temperature of 80° C. in a drying oven, granules inthe range of 50-300 μm obtained through crushing and filtering wereseparated using a sieve, and used to form the inner layer. The innerlayer was prepared in a square-shaped body of 5 cm in length and width,and 1.2-1.4 mm in thickness, and through the processing, it was preparedwith a square gasket shape of about 1 cm in width.

As shown in FIG. 1, the surface layer was prepared as a green sheet inthe range of 50-100 μm in thickness using a tape casting method.Specifically, after preparing a solution having 0.85 g of a dispersingagent (KD-2, ICI Co., Great Britain) dissolved in 38.4 g of toluene and25.6 g of ethyl alcohol, 23.9 g of alumina platelet (platelet, NanofluidCo., Korea), 3.98 g of alumina particulate (ALM 43, Sumitomo Co.,Japan), and 28.9 g of borosilicate glass (Pyrex glass, Iwaki Co., Japan)were added and ball-milled for 24 hours to prepare a uniform. A binderof 2.5 g of PVB (polyvinyl butyral) (B-97, Monsanto Co., USA), andplasticizers of 1.5 g of dibutyl phthalate and 0.5 g of polyethyleneglycol, were added to the slurry and thoroughly milled for 24 hours,then a green sheet was prepared using the tape casting method. After thegreen sheet was processed in the same matter as the inner layer, the gasleakage rate was measured by laminating with the surface layer of thehybrid sealing material. The thickness ratio of the inner layer to thesurface layer of the hybrid composite sealant was in the range ofapproximately 8-15.

Table 1 shows that the function of mechanical load applied for stackingwas compared with the gas leak rate measured when maintaining a pressuredifferential of standard atmospheric pressure within the stack comparedto the outer atmospheric pressure of the stack. In this case, themeasurement of gas leak rate was shown as a function of the mechanicalload applied for stacking, after converting changes in internal pressuremeasured when maintaining a pressure differential of standardatmospheric pressure (14.7 psi) within the stack into gas leak ratethrough the formula below.

[Formula  1] $\begin{matrix}{{L.R_{sccs}} = \frac{V \cdot \left\lbrack {{dP}_{means} - {dP}_{{no} - {leak}}} \right\rbrack}{t \cdot P_{atm}}} & \left\lbrack {{equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Herein, L.R_(SCCS) denotes the amount of gas leaked per second, Vdenotes the volume of gas, t denotes the measurement time dP_(means)denotes the pressure change during measurement, dP_(no-leak) denotes thepressure change due to parts other than the sealing material, andP_(atm) denotes standard atmospheric pressure. The amount of the gasleakage was calculated by calculating the amount of the leakage gas byunit length.

The gas leak rate estimated under the condition that only the innerlayer including alumina fiber was applied with 30 psi of externalmechanical load at a temperature of 800° C., was approximately 0.0012sccm/cm, and the thermal cycling stability was defined by the number ofthermal cycles which were required for the gas leak rate to increasebeyond an initial value by more than 20% by repeated heating and coolingat a rate of 150° C./h within the range of temperature from 400° C. to800° C. The result is shown in Table 1. The use of the hybrid compositesealant, which had excellent gas leak rate in spite of the relativelylow (30-50 psi) mechanical load applied to the compressive sealingmaterial and implementing a surface layer containing alumina platelet ofmore than 20 volume % as a reinforcement particles, showed a greatlyimproved thermal cycle durability result.

TABLE 1 Construction and characteristics of hybrid composite sealantInner layer Surface layer Gas Thermal composition (vol %) composition(vol %) Mechanical leackage cycling fibrous particulate plateparticulate pressure rate durability shape shape glass shape shape glass(psi) (sccm/cm) (times) 35 10 55 30 0.0012 16 35 10 55 30 5 65 500.0027 >50 35 10 55 10 5 90 50 0.0018 14 35 10 55 20 5 75 50 0.0020 >5035 10 55 40 5 55 50 0.0043 >50

The present invention as described provides a hybrid composite sealantlaminated with a fiber-reinforced composite inner layer and aplatelet-reinforced composite surface layer. However, without beinglimited by this, the same teaching may be applied for a hybrid compositesealant which contains fiber reinforcement particles in a glass matrixphase in an inner layer portion of the sealing material, and containsplatelet reinforcement particles in a glass matrix phase in a surfacelayer portion on either one or both sides of the inner layer portion,wherein the inner layer portion and the surface layer portions areintegrally formed.

1. A hybrid composite sealant, as a sealing material for a planar typesolid oxide fuel cell stack, having a matrix of a glass composition,wherein a surface layer reinforced with platelet reinforcement particlesis laminated onto either one or both surfaces of an inner layerreinforced with fibrous reinforcement particles.
 2. The hybrid compositesealant of claim 1, wherein the content of platelet reinforcementparticles within the surface layer is in the range of 5˜60 volume %. 3.The hybrid composite sealant of claim 1, wherein the thickness of theplatelet reinforcement particles is in the range of 0.2˜1 μm and anaspect ratio of diameter to thickness thereof is in the range of 5˜50.4. The hybrid composite sealant of claim 1, wherein the content of fiberreinforcement particles within the inner layer is in the range of 5˜55volume %.
 5. The hybrid composite sealant of claim 1, wherein the fiberreinforcement particles are in the range of 0.5˜2 μm in diameter andhave an aspect ratio of length to diameter in the range of 5-100.
 6. Thehybrid composite sealant of claim 1, wherein a thickness of each surfacelayer is at least 10 μm, and a thickness ratio of each surface layerwith respect to the inner layer is in the range of 5˜50.
 7. The hybridcomposite sealant of claim 1, wherein the inner layer may furtherinclude isotropic particulate reinforcement particles.
 8. The hybridcomposite sealant of claim 7, wherein the isotropic particulatereinforcement particles larger than the particulate reinforcementparticles included in the inner layer may further be included into theouter surface layer.
 9. The hybrid composite sealant of claim 1, whereinthe composition of glass matrix may include one of B₂O₃—SiO₂,Al₂O₃—B₂O₃—SiO₂, and CaO—B₂O₃—SiO₂.
 10. The hybrid composite sealant ofclaim 1, wherein the platelet reinforcement particles are horizontallyaligned within the surface layer.
 11. A composite sealant, as a sealingmaterial for a planar type solid oxide fuel cell stack, wherein asurface layer portion of the sealing material may include plateletreinforcement particles in a glass matrix.
 12. A hybrid compositesealant, as a sealing material for a planar type solid oxide fuel cellstack, wherein an inner layer portion of the sealing material includesfibrous reinforcement particles in glass matrix, a surface layer portionof either one or both sides of the inner layer portion includes plateletreinforcement particles in a glass matrix, and the inner layer portionand the surface layer portions are integrally formed.